1. Field of the Invention
This invention relates generally to a pump system including a variable-speed drive unit composed of a frequency converter and an electric rotary machine, and a hydraulic machine (pump, pump turbine or the like) operated at a variable speed by the variable-speed drive unit and having at least a pump function. More particularly, the invention relates to a pump system which is operable, in quick response to external command signals, by a variable speed control suited for improving the demand-supply balance of electric power of an electric power system.
2. Description of the Related Art
In conventional pumped storage power plants, it has been a common practice to perform either a generating operation or pumping-up operation, with the rotational speed of a pump turbine being fixed at a common constant value and, during pumping-up operation, linearly adjusting an opening amount on guide vanes of the pump turbine according to a predetermined function of the head.
However, the above conventional plants bear only mere loads having no degree of freedom (unadjustable), from the view point of the electric power system, during pumping-up operation to that no adjustment is possible to improve the demand-supply balance of electric power of the electric power system.
To this end, variable-speed pump systems have recently been proposed in which the rotational speed of a pump turbine is variable so as to correspond with the electrical power system during the pumping-up operation.
For example, Japanese Patent Laid-Open Publication No. 203883/1984 discloses an operating method in which, if a request was issued to reduce the input to a pump from the system while the pump is operating, the rotational speed of the pump is lowered, without creating variation or noise. This is accomplished by controlling the amount of opening of guide vanes prior to the speed control of a speed control unit of an induction generator-motor only during reduction of input to the pump.
Further, Japanese Patent Laid-Open Publication No. 128886/1985 discloses another operating method in which, if a request was issued to increase the input to a pump from the system while the pump is operating, the rotational speed of a pump is increased, without creating vibration of noise, by controlling the speed change of an induction generator-motor prior to the control of opening of the guide vanes for adjusting the amount of water flow, only during increase of input to the pump.
However, these two prior operating methods aim primarily at the stability of operation of a pump and vary the rotational speed of the pump within the range, in which the pump is stably operated, in an attempt to meet the request from the electric power system. Since this elongates the total response time of the system more than necessary, it is impossible to quickly respond to rapid fluctuations of the electric power system.
To cope with this prior problem, a solution was proposed by Japanese Patent Laid-Open Publication No. 212774/1988 disclosing a technical concept of controlling a variable-speed pumping-up generator to respond quickly to the fluctuation of the electric power system.
The variable-speed pump system of the Japanese Publication No. 212774/1988, as shown in FIG. 17 of the accompanying drawings, comprises a variable-speed drive unit composed of a frequency converter 3 and an electric motor 2, a pump 4 for being operated at a variable speed by the variable-sped drive unit, and a control system for controlling the variables-speed drive unit and the pump 4.
The control system of this prior pump system is equipped with a rotational speed control circuit, an electric power control circuit, and a guide vanes control circuit.
The rotational speed control circuit includes an optimum rotational speed function generator 12 for inputting an electric power command signal P.sub.O from outside and a static head HST (representing a water level difference of upper and lower reservoirs of the pump) at that time and then calculating an optimum rotational speed N.sub.OPT at that time from these inputs. The rotational speed control circuit further includes adder 18 which compares an output signal N.sub.OPT from the function generator 12 with a real rotational speed N and constitutes a negative feedback circuit, and an electric power control correction signal generator 16 equipped with at least an integrating element for nullifying the difference between the output signal N.sub.OPT and the real rotational speed N. Accordingly the rotational speed control circuit outputs a correction signal .epsilon..
The electric power control circuit includes an adder 19 for summing an electric power command signal P.sub.O and its correction signal .epsilon. to obtain a composite signal P.sub.O +.epsilon., and an adder 20 which compares this composite signal with a real input PM and constitutes a negative feedback circuit, and an electric power controller 7 equipped with at least an integrating element for nullifying the difference between the composite signal and the real input P.sub.M. Accordingly the input P.sub.M of the electric motor 2 is controlled by the frequency converter 3, which is for alternating excitation, so as to be a composite signal P.sub.O +.epsilon..
The guide vanes control circuit includes an optimum amount-of-opening-of-guide-vanes signal generator 13 for inputting a load command signal P.sub.O from outside and a static head HST and then calculating an optimum amount of opening of the guide vanes Y.sub.OPT at that time from these inputs, an adder 21 which compares an output signal Y.sub.OPT of the signal generator 13 with a real amount of opening of the guide vanes Y and constitutes a negative feedback circuit, and guide vanes controller 9 for nullifying the difference between the output signal Y.sub.OPT and the real amount of opening of the guide vanes by an integral element built in the controller 9.
With this prior control system, it is possible to take in equilibrium state N=N.sub.OPT by the rotational speed control circuit, normally P.sub.M =P.sub.O +.epsilon. by the electric power control circuit, and Y=Y.sub.OPT by the guide vanes control circuit. Since the difference between an input P.sub.p requested by the pump and a real input P.sub.M to the electric motor is absorbed to zero by the integrating action of a moment of inertia GD.sup.2, which is normally regarded as a sort of integrating element and is possessed by the electric motor 2 and the pump 4, P.sub.M =P.sub.p. Further, if errors of the function generators 12, 13 are neglected, substantially Y.sub.OPT =Y corresponding to P.sub.O and N.sub.OPT =N corresponding to P.sub.O. Therefore the relation P.sub.O =P.sub.p =P.sub.M =P.sub.O =.epsilon.is established so that the correction signal .epsilon. for electric power control will be zero.
According to this prior technology, therefore, it is possible to control the real input P.sub.M in accordance with the electrical power command signal P.sub.O from outside. This prior technology is adapted to variable-speed pumps of the type in which the input of the electric motor responds slowly to the fluctuation of the electric power system, and particularly in such a manner that the rotational speed and the amount of opening of the guide vanes can virtually follow. However, this prior technology cannot be adapted to variable-speed pumps of the type in which the input of the electric motor is controlled quickly to respond to the fluctuation of electric power of the electric power system, and particularly with a negligibly small time constant, compared to the time constant of the rotation system dominated by a moment of inertia. This is because the difference between the quickly varying input of the electric motor and the rotational speed of the pump, as well as the varying speed of the amount of opening of the guide vanes, which follow that input, become too large in transient states so that dissociation would occur among the input of the electric motor, the rotational speed of the pump, and the amount of opening of the guide vanes.
Otherwise, assuming that the rotational speed of the pump and the response speed of the amount of opening of the guide vanes are increased in an attempt to avoid the dissociation, the balance between the rotational speed of the pump and the amount of opening of the guide vanes will be lost so that the pump would possibly fall into the partial reverse flow characteristic (hump characteristic) region.
Therefore, in the prior art, there is no disclosure or suggestion of the basic principles of the system in which the input of the electric motor is controlled with a negligibly small time constant, compared to the time constant of the rotation system, and the above described dissociation is reduced to a minimum, to avoid any partial reverse flow characteristic of the pump. In other words, there is no disclosure or suggestion of the basic principles of avoiding the reverse flow characteristics and reducing the control time constant of the motor input to a minimum.